Referring to FIG. 1, a prior art bicycle is illustrated. The bicycle has a frame, a front wheel and a rear wheel. The diameters of the front wheel and rear wheel are about 1 meter. The wheels are driven by pedals attached to the frame. The direction of the bicycle is controlled by the front wheel. The prior art bicycle is heavy and large and thus to be suitable for the sizes of children and adults, various styles are designed. As a result, a children must change a new one with the increasing of age. Moreover, the bicycle easily falls down.
Referring to FIG. 2, a prior art tricycle is shown. In this kind of tricycle, the rear wheel in bicycle is changed to two small wheels. The front wheel is driven and is as a direction controller.
Some foldable bicycles have been developed, but they are inconvenient as it is folded (the operation must have some mechanical knowledge). Moreover, it is difficult in transfer.